1. Field of the Invention
This present invention relates to a method of processing seismic data and, in particular, to a method of processing marine seismic data to, reduce the effects of “ghost reflections”. The present invention can be applied to processing both marine seismic data acquired in a flat sea and marine seismic data acquired in a rough sea. The present invention also relates to processing seismic data so as to estimate the depth of a seismic receiver, or so as to monitor the accuracy of a seismic receiver.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a marine seismic survey in which seismic energy is emitted from a source 1 and detected by a seismic receiver 2 at a depth h below the surface 6 of the sea. Energy emitted from the source is reflected by the seabed 3 or by a reflector 4 below the seabed 3 and is then detected by the receiver. This path of seismic energy is labelled 5 in FIG. 1. Information about the geological structure of the earth's interior can be derived from the reflected seismic energy incident on the receiver.
The seismic receiver 2 shown in FIG. 1 is a streamer, which is a type of seismic receiver that is frequently used in marine seismic surveying. A streamer contains a plurality of sensors S1, S2 . . . SN such as pressure sensors and/or particle velocity sensors distributed along its length, which may be some hundreds of meters, and is thus able to measure the reflected seismic energy at a number of points simultaneously. A streamer is generally suspended from one or more floats 8, so that all the receivers of the streamer are at the same depth in a flat sea.
In addition to the desired path 5 of seismic energy shown in FIG. 1, other seismic energy paths will occur as a result of seismic energy being reflected or scattered from the sea surface 6. These paths are known as “ghost reflections”. For example, reference 7 in FIG. 1 shows a ghost reflection in which the seismic energy reflected by the reflector 4 is not directly incident on the receiver 2, but undergoes a further reflection at the sea surface 6 before reaching the receiver. Down-going sea-surface ghost reflections are an undesirable source of contamination of seismic data, since they obscure the interpretation of the desired up-going reflections from the earth's interior.
Ghost reflections also arise from sea surface reflections or scattering near the source 1. The overall seismic energy signal received at a point below the source will contain a component that has come direct from the source and a component that has come via a reflection or scattering at the sea surface.